In recent years, from the viewpoint of global environment conservation, the improvement of fuel efficiency of automobiles has been a critical issue. Development in which an increase in the strength of materials used for automobile bodies reduces thicknesses to lighten automobile bodies has been actively made.
To increase the strength of a steel sheet, in general, it is necessary to increase proportions of hard phases such as martensite and bainite with respect to all microstructures of the steel sheet. However, an increase in the strength of the steel sheet by increasing the proportions of the hard phases causes a reduction in workability. Thus, the development of a steel sheet having both high strength and good workability is required. Hitherto, various composite-microstructure steel sheets, such as ferrite-martensite dual phase steel (DP steel) and TRIP steel utilizing transformation-induced plasticity of retained austenite, have been developed.
In the case where the proportions of the hard phases are increased in a composite-microstructure steel sheet, the workability of the hard phases strongly affects the workability of the steel sheet. The reason for this is as follows: In the case where the proportions of the hard phases are low and where the proportion of soft polygonal ferrite is high, the deformation ability of polygonal ferrite is dominant to the workability of the steel sheet. That is, even in the case of insufficient workability of the hard phases, the workability such as ductility is ensured. In contrast, in the case where the proportions of the hard phases are high, the workability of the steel sheet is directly affected not by the deformation ability of polygonal ferrite but by deformation abilities of the hard phases.
Thus, in the case of a cold-rolled steel sheet, the workability of martensite is improved as follows: Heat treatment for adjusting the amount of polygonal ferrite formed in the annealing step and the subsequent cooling step is performed. The resulting steel sheet is subjected to water quenching to form martensite. The steel sheet is heated and maintained at a high temperature to temper martensite, thereby forming a carbide in martensite as a hard phase. However, such quenching and tempering of martensite require a special manufacturing apparatus such as a continuous annealing apparatus with the function to perform water quenching. Thus, in the case of a usual manufacturing apparatus in which a steel sheet cannot be heated again or maintained at a high temperature after the hardening of the steel sheet, although the steel sheet can be strengthened, the workability of martensite as a hard phase cannot be improved.
As a steel sheet having a hard phase other than martensite, there is a steel sheet having a main phase of polygonal ferrite and hard phases of bainite and pearlite, in which bainite and pearlite as the hard phases contain carbide. The workability of the steel sheet is improved by not only polygonal ferrite but also the formation of carbide in the hard phases to improve the workability of the hard phases. In particular, the steel sheet has improved stretch-flangeability. However, since the main phase is composed of polygonal ferrite, it is difficult to strike a balance between high strength, i.e., a tensile strength (TS) of 980 MPa or more, and workability. Furthermore, in the case where the workability of the hard phases is improved by forming carbide in the hard phases, the workability of the resulting steel sheet is inferior to the workability of polygonal ferrite. Thus, in the case of reducing the amount of polygonal ferrite to achieve a high tensile strength (TS) of 980 MPa or more, sufficient workability cannot be provided.
Japanese Unexamined Patent Application Publication No. 4-235253 discloses a high-strength steel sheet having good bendability and impact resistance. The microstructure of that steel sheet is fine uniform bainite including retained austenite obtained by specifying alloy components.
Japanese Unexamined Patent Application Publication No. 2004-76114 discloses a composite-microstructure steel sheet having good bake hardenability. Microstructures of that steel sheet contain bainite including retained austenite obtained by specifying predetermined alloy components and the retained austenite content of bainite.
Japanese Unexamined Patent Application Publication No. 11-256273 discloses a composite-microstructure steel sheet having good impact resistance obtained by specifying predetermined alloy components and the hardness (HV) of bainite to form microstructures containing 90% or more bainite including retained austenite in terms of the proportion of area and 1%-15% retained austenite in bainite.
However, the steel sheets described above have the problems described below. For example, in the component composition described in JP '253, it is difficult to ensure the amount of stable retained austenite that provides a TRIP effect in a high-strain region when strain is applied to the steel sheet. Although bendability is obtained, ductility until plastic instability occurs is low, thereby leading to low punch stretchability.
In the steel sheet described in JP '114, bake hardenability is obtained. However, in the case of providing a steel sheet having a high tensile strength (TS) of 980 MPa or more or 1050 MPa or more, it is difficult to ensure the strength or workability such as ductility and stretch-flangeability when the steel sheet has increased strength because the steel sheet mainly contains bainite or bainite and ferrite and minimizes martensite.
The steel sheet described in JP '273 aims mainly to improve impact resistance. The steel sheet contains bainite with a hardness HV of 250 or less as a main phase. Specifically, the microstructure of the steel sheet contains more than 90% bainite. Thus, it is difficult to achieve a tensile strength (TS) of 980 MPa or more.
It could therefore be helpful to provide a high-strength steel sheet having good workability, in particular, ductility and stretch-flangeability, and having a tensile strength (TS) of 980 MPa or more, and to provide an advantageous method for manufacturing the steel sheet.